Gasification is a partial oxidation process wherein carbon feedstock are converted into CO and H2 (often known as synthesis gas) in presence of steam and oxygen. These reactions occur typically above 800 degree C. High reaction temperatures and steam partial pressures are often preferred for maximizing H2 production. Currently there are three major types of gasification systems. These are Entrained bed, fluidized bed and Moving bed gasifiers. The entrained bed system operates at temperatures above 1250 degree C., while in fluid bed and moving bed gasifiers the temperatures are normally below 1100 degree C. Accordingly, the solids residence time is much higher in these systems.
Though all gasification systems convert solid carbonaceous material into gaseous form, choosing gasifier configuration based on the properties of feedstock can result in improved operational performance and higher conversion efficiency. For example, high ash coals are often advantageously gasified in moving bed systems to achieve higher cold gas efficiencies. Similarly, petroleum coke is typically gasified in entrained beds to overcome the reactivity issues at low temperatures. Also the size of the particles used in these gasifiers differs significantly to suit the flow pattern and operating conditions employed in the gasifier. Similarly in entrained bed gasifiers the temperatures are typically maintained above the ash fusion temperatures. Therefore physical properties like grindability index, ash melting temperatures etc play important role in deciding optimal gasifier choice.
The syn gas quality and its calorific value are the vital issues of gasification, which is mainly governed by the type of feedstock used and operating conditions employed. Since feedstock plays an important role, several studies focused on co-processing of coal, pet coke, biomass, heavy liquids etc. However, co processing two or more types of feedstocks in a single gasifier configuration can be inefficient compared to co processing in multi stage gasifier configuration as single system can not exploit all the beneficial characteristics of the feed.
The idea of gasifying carbonaceous material in different stages has been reported in many patents and patent applications. However, most of these technologies utilize two stage systems for increasing the carbon conversion by converting the unconverted carbon of first stage in the second stage or emphasize on effectively removing tar from gasification system. U.S. Pat. No. 7,740,672, discloses a two stage gasifier wherein the two stages of the gasifier are connected to a cyclone separator in between and the cold gas efficiency of the effluent gases is expected to be around 90%.
U.S. Pat. No. 4,872,886 discloses a two stage gasification process where a part of the coal feed is reacted in the presence of steam and in an oxygen rich environment to produce syngas at a very high temperature. This hot syngas is then contacted with a second part of coal slurry where further gasification reaction takes place. Unconverted coal is recycled back to the first stage for combustion. By doing this high calorific value syngas is generated with minimum energy loss. This increases the overall efficiency of the process. But this process only discloses the gasification of a single feedstock in two stages. In second stage the unconverted feedstock get reacted with the syn gas produced in the first stage to give synthesis gas.
U.S. Pat. No. 7,503,945 describes a two stage gasification process wherein the solid effluents coming out of the first stage gasifier are combusted in an ash reactor. The hot gases coming out of the ash reactor is fed to a cyclone separator wherein sufficient amount of ash particles are allowed to pass along with gas effluent which in turn mixes with the gaseous effluent of the first stage. By doing this tar coming out of the first stage is being picked up by ash and taken to the ash reactor.
The idea of supplying the heat for carrying out endothermic gasification reaction using the sensible heat of steam has been disclosed in WO2009145724. This application discloses a two step gasification process for converting carbonaceous feedstock into gaseous syngas by supplying sensible heat from external sources.
None of these two stage processes discuss the preferential combustion in case of co-gasification of two or more carbonaceous feedstocks. United States patent application 2010/146856 suggests carrying out gasification in two zones through preferential gasification of high calorific material in zone 1 and low calorific material in zone 2. However, this invention does not teach any optimum scheme for reduced oxygen consumption while handling feedstock with differing ash, hydrogen and calorific content. Hydrogen rich feedstocks such as methane, petroleum residue consume less oxygen and release higher amount of gasification reactants such as H2O and CO2. The feedstock that are rich in hydrogen are high in H/C ratio, hence produces more stoichiometric H2O during combustion than hydrogen lean feedstock. As the kinetics of steam gasification are much faster than the CO2 gasification, preferential combustion of high hydrogen content feeds enables faster kinetic environment for carbon conversion. Further, if the oxygen supply is limited to the extent of gasification conditions, the calorific content of the fuel is not fully available for driving the endothermic gasification reactions. This is because under high temperature conditions both CO and CO2 are produced leading to less enthalpy release as energy is locked in the form of CO and H2. H2 is produced from CO and H2O through shift reaction. As a result the mixed temperatures achieved while co-gasification are lower. Therefore stoichiometric gasification of high calorific value fuel is not optimal for creating high temperature, steam rich gasification environment.
Ash content in the feed consumes significant energy which is often wasted in the form of heat of fusion or sensible heat. This is more so when combustion is carried out in a separate reactor where the scope for sensible heat transfer from ash is limited. In contrast, when low ash feedstocks are burnt the temperature rise is much higher due to absence of high ash levels. As a result all the energy can be efficiently captured in the form of high enthalpy stream mainly consisting of H2O and CO2.
Further, the advantages of segregated treatment are not fully exploited as these feedstocks are completely mixed in a single reactor system.
To improve the system further, the present invention includes preferential combustion of a carbonaceous material which has higher calorific content preferably higher hydrogen and low ash content calorific value in a reactor and transferring this energy using steam for driving the gasification of lower calorific value ash rich and hydrogen lean carbonaceous feed in a separate reactor. More particularly, the present invention relates to a two stage gasification system and process for simultaneous or co-gasification of two or more carbonaceous feedstock.